The present invention relates to a variable rotational speed oil free screw compressor and a control method therefor and, more particularly, to a variable rotational speed oil free screw compressor having a low-pressure stage compressor body and a high-pressure stage compressor body and a control method therefor.
In a conventional variable rotational speed oil free screw compressor, for example, as disclosed in Japanese Patent Laid-Open No. 82391/1998, a low-pressure stage screw compressor body and a high-pressure stage screw compressor body are connected to each other in series, and a cooler is provided between the two compressing sections. A motor is connected to each of the low-pressure stage screw compressor body and the high-pressure stage screw compressor body, and the motor is driven at a variable speed by an inverter. In the variable rotational speed oil free screw compressor constructed above, with a small flow rate, both of the low-pressure stage screw compressor body and the high-pressure stage screw compressor body rotate at low speed, so that an amount of internal leakage cannot be ignored. Therefore, a blow-off valve is connected to an outlet pipe, by which blow-off control is carried out while the low-pressure stage screw compressor body and the high-pressure stage screw compressor body are operated at the lowest rotational speed.
With an oil free screw compressor having two stages of low-pressure and high-pressure stages, power consumption with no load is smaller than that with full load as compared with a single-stage oil free screw compressor. Therefore, even if the method disclosed in the above-described Publication is applied at the time of no-load operation, there is a disadvantage that power consumption is not decreased so much as compared with a conventional method, in which a suction throttle valve is throttled.
The present invention has been contrived in view of the above problems of the prior art, and has its object to provide a variable rotational speed oil free screw compressor having a low-pressure stage compressor body and a high-pressure stage compressor body, in which power consumption is reduced both at no load and at low load.
A first feature of the present invention for attaining the above object is a variable rotational speed oil free screw compressor, comprising a low-pressure stage compressor body and a high-pressure stage compressor body, which are variable in rotational speed, and blow-off means for blowing off compressed air to the atmosphere midway a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body. In this feature, it is preferable that an inter-cooler and blow-off means is provided midway the pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body, and an after-cooler is provided on a discharge side of the high-pressure stage compressor body. Also, another blow-off valve may be provided between the high-pressure stage compressor body and a check valve to blow off compressed air discharged from the high-pressure stage compressor body, and wherein compressed air is blown off through the blow-off valve and another blow-off valve at the time of no-load or low-load operation.
Also, a pressure detector may be provided on the discharge side of the high-pressure stage compressor body for detecting pressure of high-pressure air discharged from the high-pressure stage compressor body, and a controller may be provided to receive a signal of discharge pressure detected by the pressure detector and output a control signal for controlling the blow-off means. The variable rotational speed oil free screw compressor may further comprise an electric motor for rotatingly driving the low-pressure stage compressor body and the high-pressure stage compressor body, and an inverter for driving the electric motor, and wherein the controller controls the inverter based on a signal of discharge pressure detected by the pressure detector.
Preferably, a suction throttle valve is provided on a suction side of the low-pressure stage compressor body, and another blow-off means is provided on a discharge side of the high-pressure stage compressor body, another blow-off means interconnecting with the suction throttle valve.
A second feature of the present invention for attaining the above object is a method of controlling a variable rotational speed oil free screw compressor adapted to operate in accordance with a volume of consumed air on a usage side while changing rotational speeds of a low-pressure stage compressor body and a high-pressure stage compressor body, the method comprising the steps of: performing a load operation to change rotational speeds of the low-pressure stage compressor body and the high-pressure stage compressor body in a region, in which a volume of consumed air based on pressure detected by a pressure detector provided on a discharge side ranges from a maximum air volume to a preset volume of air; operating the low-pressure stage compressor body and the high-pressure stage compressor body at set lower limit rotational speeds preset every compressor body in a no-load operation, in which a volume of consumed air is substantially zero, and blowing off compressed air from blow-off means provided in a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body; and repeating the load operation and the no-load operation when a volume of consumed air is equal to or smaller than a set air volume.
Preferably, during the load operation, the rotational speeds of the low-pressure stage compressor body and the high-pressure stage compressor body are changed substantially in proportion to a volume of consumed air. Also preferably, during no-load operation, compressed air discharged from the high-pressure stage compressor body is blown off.
A third feature of the present invention for attaining the above object is a method of controlling a variable rotational speed oil free screw compressor adapted to operate in accordance with a volume of consumed air on a usage side while changing rotational speeds of a low-pressure stage compressor body and a high-pressure stage compressor body, the method comprising the steps of: performing operations including a no-load operation to blow off compressed air from blow-off means provided in a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body when a volume of consumed air based on pressure detected by a pressure detector provided on a discharge side is not exceeding a preset volume of air and pressure of compressed air discharged from the low-pressure stage compressor body is at least the atmospheric pressure.
A suction throttle valve provided on a suction side of the low-pressure stage compressor body may be made to interlock with blow-off means for compressed air compressed by the high-pressure stage compressor body to make control to throttle the suction throttle valve when a volume of consumed air is not exceeding a set air volume.